apple-touch-icon

Apple's scheme allows a site to offer images of different sizes so
the client can choose the most appropriate one according to its
screen size and resolution.

Apple devices and applications completely ignore the favicon.ico
de facto standard which, while somewhat quirky in its use of the
ico format, has been pretty much universally adopted elsewhere.
Conversely, apple-touch-icon.png will be ignored by non-Apple
devices, possibly because its 16x16 resolution would look pretty
shabby on most smart phones.